In recent years, remarkable progress of miniaturization of semiconductor devices has been made, and its processing rule has fallen under the realm of wavelength of light for exposure. With the progress, a new problem of the difference in size of the resist has arisen. The difference in size occurs between a dense pattern area having many gathered patterns, and an isolated pattern area having only one pattern, since optical proximity effect occurs in the dense pattern area and does not occur in the isolated pattern area. This difference in size of the resist degrades performance of the semiconductor devices, and further, it makes it more difficult to ensure high definition of resolution pattern and causes a problem to occur in larger-scale integrated circuit of semiconductor pattern. Note that, the optical proximity effect will be described in detail later.
Further, recently, a half tone mask has been created which uses a half tone film instead of a chrome film. The half tone film has a transmittancy of 1 to 25%, and provided with shifting function for shifting the phase of transmission light through the film to be off-phase by 180° from the phase of transmission light in a light transmission section (opening section). The half tone mask can provide a higher resolution compared to the case of adopting a chrome film. Japanese Unexamined Patent Publications Tokukaihei 8-328235 (published on Dec. 13, 1996), Tokukaihei 9-50116 (published on Feb. 18, 1997) and some other Japanese Patent publications disclose the half tone mask.
Here, the following will explain the difference between light for exposure when adopting a half tone mask, and light for exposure when adopting a conventional chrome mask.
As shown in FIG. 7(a), in the case of the chrome mask in which a chrome film 55 is formed on a transparent substrate 51, since the chrome film 55 is totally lightproof, light is only transmitted through an opening section 56 (light X) where the transparent substrate 51 is exposed.
On the other hand, as shown in FIG. 7(b), in the case of the half tone mask in which a half tone film 52 is formed on the transparent substrate 51, light is transmitted through the opening section 56 (light X) where the transparent substrate 51 is exposed, and also, a little quantity of light is transmitted through the half tone film 52 (light Y) by being adjusted its phase to be off-phase by 180° from that of the light transmitted through the opening section 56. The light Y has transmittancy of several %, and is in opposite phase of that of the light X. The light X and the light Y compose the light Z which is irradiated to a resist. Since the light Z is composed of the light X and light Y which are in opposite phases, the light Z (light for exposure) draws a sharp curve in a light intensity distribution, thereby providing a higher resolution compared to the case of adopting a chrome film shown in FIG. 7(a).
Note that, it is most effective that the phase of the transmission light through the half tone film 52 is adjusted to be off-phase by 180° from that of the light transmitted through the transparent substrate 51; however, resolution can be raised when it is adjusted to be within a range of 180°±10.
The following will explain a conventional half tone mask used for the manufacturing of a semiconductor device with reference to drawings.
FIG. 8(a) shows a conventional half tone mask. Note that, the cross-sectional view of the mask in FIG. 8(a) is taken along the line A—A in the upper view of the mask. As described above, the half tone mask has a structure in which a half tone film 52 is selectively formed on the transparent substrate 51. The transparent substrate 51 is normally made of quartz of 6 inches square having the thickness of 0.25 inch. The half tone film 52 is made of molybdenum silicide, and the thickness is adjusted to 90 to 110 nm so as to have an optical characteristic of 5 to 6% transmittancy. In FIG. 8(a), which is an upper view of the half tone mask, the area of transparent substrate 51 is a light transmission section (opening section) for transmitting light, and the area of half tone film 52 is a light blocking section.
When a pattern is transferred to a resist (photo resist) by using such a half tone mask with an exposure device, the optical proximity effect occurs in a dense pattern area. The optical proximity effect (shown in FIG. 8(b) is distortion of the light intensity distribution of adjacent transmission light in the dense pattern area which causes the difference in size of resist between the dense pattern area and the isolated pattern area and occurrences of roundness or over cutting of the edge of the pattern. This effect becomes more prominent with high definition pattern, and also, as the resist becomes thicker. Note that, the light intensity distribution in FIG. 8(b) is taken at the line B—B in the upper view of the mask.
When the optical proximity effect is occurred, the dense pattern area having many gathered patterns and the isolated pattern area having only one pattern become different to each other in size on the wafer, though these areas are the same in size on the mask. For example, in FIG. 8(b), the hole size of the created contact hole 57 varies in the dense pattern area and in the isolated pattern area even though the light transmission area, i.e., the opening sections 56 disposing the transparent substrate 51 are the same in size in both areas (refer to FIG. 8(a)). The hole size C in the dense pattern area is larger than the hole size A in the isolated pattern area.
FIG. 9 shows pitch dependence of the hole size in the dense pattern area, in the case where a pattern is transferred to a resist by using a conventional half tone mask with an exposure device of ArF laser (wavelength: 193 nm). In the figure, the opening section 56 of the conventional half tone mask is formed as a rectangle of 190 nm×190 nm. Further, FIG. 10 shows the mask pattern of the conventional half tone mask used for the measurement of the hole size, and further, FIG. 11 shows a light intensity distribution curve in the exposure at each pitch. In FIG. 10, W indicates the pitch of the opening section 56.
FIG. 9 revealed that the hole size gradually becomes larger as the pitch of the opening section 56 becomes at or smaller than 500 nm. This is because, as the pitch becomes smaller, the area of the half tone film 52 becomes narrower. This decreases phase shifting effect by the half tone film 52, and decreases resolution. The hole size becomes the largest when the pitch is 380 nm, and then, it starts to gradually become smaller as the pitch becomes further smaller. This is because, as shown in FIG. 11, the decrease of the phase shifting effect is saturated when the pitch becomes smaller than 380 nm, and now the maximum value of the intensity of the light for exposure starts to decrease.
As described, when the difference in size of the resist is occurred between the dense pattern area and the isolated pattern area due to the optical proximity effect during the manufacturing of the semiconductor device, the difference in size of the resist degrades performance of the semiconductor devices, and further, it makes it more difficult to ensure high definition of resolution pattern and causes a problem to realize a larger-scale integrated circuit of semiconductor pattern.
Japanese Patent Publication Tokukaihei 9-246149 (published on Sep. 19, 1997) teaches a technique for suppressing the transmittancy of the light for exposure in the dense pattern area, in order to correct the difference in size of the resist between the dense pattern area and the isolated pattern area due to the optical proximity effect.
The mask device disclosed in the publication above has an arrangement such that a circuit pattern is transferred to a chrome pattern on the upper surface of a transparent quarts substrate, and is equipped with a light quantity suppressing film in the dense chrome pattern area, where the wavelength of the light for exposure is at or 4 times of wiring pitch, for suppressing the transmittancy of the light for exposure. The light quantity suppressing film is provided either on a front surface or a rear surface of the quartz substrate. The light quantity suppressing film has a transmittancy of not less than 85% but less than 97% of the amplitude transmittancy of the light for exposure, and not less than 72% but less than 94% of the light intensity transmittancy of the light for exposure.
According to the publication, by having such a light quantity suppressing film, the light intensity in the area having dense layout patterns is suppressed, and also processing size variation due to the optical proximity effect can also be suppressed.
However, as with the mask device described in the foregoing publication, in the method of forming the light quantity suppressing film having transmittancy of 72% but less than 94% on the whole area of the dense pattern area, the exposure have to be performed in a longer period since light intensity of whole area decreases. Thus, it poses a problem of increasing the manufacturing cost.